The present invention relates to a device for preventing an exhaust gas recirculation valve for an internal combustion engine, in particular for a motor vehicle, from sticking, having a recirculation line which branches off from an exhaust line and leads into a fresh gas line and is used for recirculating exhaust gas, the recirculation line having a valve with a closing element and the closing element being able to be moved between a closing position and an opening position via an actuator, and when the internal combustion engine is not in operation, the closing element of the valve being placed in an idle position by means of the electric motor.
In an internal combustion engine, a fuel/air mixture is fed to the internal combustion engine, burned there and subsequently led out of the internal combustion engine again as exhaust gas. The exhaust gas led out of the internal combustion engine is usually carried off into the surroundings, but some of it may be fed back to the internal combustion engine. Recirculation of the exhaust gas back into the internal combustion engine takes place by means of an exhaust gas recirculation device. An exhaust gas recirculation device is used to remove some of the exhaust gas from the exhaust line, add it again to the fresh gas line and feed it back to the internal combustion engine.
The recirculation of exhaust gas means that the cylinders of the internal combustion engine obtain, given the same volume of gas, a smaller amount of fresh fuel/air mixture than would be the case if the exhaust gas were not added to the fuel/air mixture. Adding exhaust gas to the fuel/air mixture results in a reduction in the combustion temperature in the cylinders of the internal combustion engine because the recycled exhaust gas no longer actively participates in the combustion.
At a low combustion temperature in comparison to a high combustion temperature, fewer nitrogen oxides are produced and the proportion of unburned hydrocarbon compounds or CH compounds increases. The proportion of unburned hydrocarbon compounds and the fuel consumption which may increase determine the upper limit of the exhaust gas recirculation rate. For example in the case of direct injection diesel engines when idling, the exhaust gas recirculation rate can be up to 50%. In addition, given a particularly high recirculation of exhaust gas, the quiet running of the internal combustion engine may deteriorate.
In the operating ranges of the internal combustion engine in which the internal combustion engine is to be operated with a particularly rich fuel/air mixture, for example at full load, the exhaust gas recirculation device is turned off. In this case, a rich fuel/air mixture means that significantly less air is actually sucked in than is theoretically required for complete combustion. Correspondingly, a lean mixture also refers to an excess of air.
The recirculation of exhaust gas conventionally takes place via an exhaust gas recirculation line which connects the exhaust line to the fresh gas line. In this case, the exhaust gas recirculation line can branch off from the exhaust line directly after the engine and before the exhaust. For a variable rate of recirculation of exhaust gas, a valve is frequently fitted into the exhaust gas recirculation line. The closing element of the valve can conventionally be activated by a control unit as a function of different parameters, for example rotational speed of the engine and engine temperature.
During operation of the internal combustion engine and of the exhaust gas recirculation device, the exhaust gas recirculation line and the valve seat and the closing element of the valve, inter alia, become clogged due to impurities in the exhaust gas. During operation of the internal combustion engine, these impurities form a virtually liquid film on the walls of the exhaust gas recirculation line, an the valve seat and on the closing element of the valve. When the internal combustion engine is switched off, cooling or cooled impurities from the exhaust gas are deposited on the walls of the exhaust gas recirculation line, on the valve seat and on the closing element of the valve.
Due to the impurities forming layers on the exhaust gas recirculation line, on the valve seat and on the closing element of the valve, the closing element of the valve can adhere to the valve seat or to the exhaust gas recirculation line when the internal combustion engine is switched off. This is because, when the internal combustion engine is switched off, the closing element of the valve is conventionally moved into its closing position. The effect of the closing element of the valve adhering to its valve seat or to the exhaust gas recirculation line due to deposited impurities from the exhaust gas may be that the valve can no longer be opened by the means provided for this, for example an electric motor, upon renewed operation of the internal combustion engine. Operation of the exhaust gas recirculation device is no longer possible with a closing element of the valve adhering to the valve seat. Re-starting of the exhaust gas recirculation device is only possible following cleaning of the closing element of the valve and of the valve seat and of the exhaust gas recirculation line.
DE-A-198 25 583 discloses an exhaust gas recirculation device of the type mentioned at the beginning, in which the closing element is placed in its idle position by means of a spring.
DE-A-198 01 383 likewise discloses an exhaust gas recirculation device of the type mentioned at the beginning, in which, when the ignition is switched off, an engine controller activates the drive of the closing element in such a manner that the closing element is moved into its opening position and is blocked there.
The invention is based on the object of specifying a device of the type mentioned above for an internal combustion engine, in which, with a particularly small outlay on production, the closing element of the valve is particularly reliably prevented from adhering to the valve seat or to the exhaust gas recirculation line.
This and other objects are achieved according to the invention in that, the closing element of the valve can be activated by the actuator via a gear mechanism, it being possible for a rotational movement of the actuator to be converted into a linear movement of the closing element of the valve by means of the gear mechanism, and in that the gear mechanism has a first movement range and a second movement range, the first movement range being limited by the opening position and the closing position of the closing element of the valve, and the second movement region being limited by the closing position and the idle position of the closing element of the valve.
The invention is based on the consideration that an exhaust gas recirculation device which is to be produced with a particularly low outlay on production and in which, at the same time, the closing element of the valve is particularly reliably prevented from adhering to the valve seat or to the exhaust gas recirculation line, is to have a particularly small number of components. Therefore, as small as possible a number of additional elements should be required in order to reliably prevent the closing element of the valve from adhering to the valve seat or to the exhaust gas recirculation line even when the internal combustion engine is not in operation. The xe2x80x9ctrailing effectxe2x80x9d of the control apparatus can therefore be used to make a movement of the closing element of the valve possible by means of an actuator even after the internal combustion engine is switched off. The time-delayed turning off of the electronic control unit after the ignition of the internal combustion engine has been switched off enables the closing element of the valve to be shifted, when the internal combustion engine is not in operation, into an xe2x80x9cidle positionxe2x80x9d, in which the valve is at least partially open. The idle position differs with the closing position and the opening position of the closing element of the valve. It is situated between the completely closed and completely open states of the closing element of the valve.
The use of a gear mechanism makes it possible to use a standard electric motor as actuator, as a result of which the outlay on costs for the exhaust gas recirculation device are particularly low. In addition, the movement of the closing element of the valve and therefore the region between the opening position and closing position of the closing element of the valve can be particularly finely adjusted by means of the gear mechanism.
The definition of an idle position for the closing element of the valve, which position lies outside the customary movement range of the closing element of the valve, enables this idle position to be configured individually as a function of the vehicle type, size of the engine and impurities from the fuel which is used.
The idle position of the closing element of the valve is advantageously predetermined by a stop for the gear mechanism. A stop may also be adjusted subsequently, for example during maintenance work on the motor vehicle and on the internal combustion engine, thus enabling the exhaust gas recirculation device to be adapted in a particularly simple manner to wear and changed operating conditions. In this connection, it proves particularly advantageous if the stop can be adjusted by itself.
The gear mechanism for resetting the closing element both into the idle position and into the closing position advantageously has a resetting spring. A resetting spring, which is stressed when the closing element of the valve is opened and, when the valve is open, pushes the closing element back counter to the force of the actuator into the closing position or the idle position, can move the closing element of the valve into the closing position should the actuator fail. A prerequisite for this is that the actuator can be moved by the force of the resetting spring. The resetting spring particularly reliably ensures that, should the actuator fail, the closing element of the valve is moved into the closing position or is fixed in the idle position. With a defective actuator, the internal combustion engine can be used, albeit without the exhaust gas recirculation device, but otherwise correctly.
The current position of the closing element of the valve can advantageously be detected by a sensor connected to a control device. This makes it possible, in all operating states of the exhaust gas recirculation device, to check where the closing element of the valve is currently placed. This enables the respectively current position of the closing element to be particularly finely coordinated with other parameters of the internal combustion engine.
The valve is advantageously a disk valve. A disk valve has an approximately disk-shaped closing element with a retaining element. A corresponding cutout in a pipe wall can be particularly reliably covered by a disk valve. For complete covering of the pipe wall by the disk valve, there can be sealing elements between the pipe wall and the approximately disk-shaped closing element. The approximately disk-shaped closing element can be moved either into the pipe for opening purposes or else out of the pipe for opening purposes.
The amount of fresh gas to be supplied to the internal combustion engine can advantageously be set via an activatable swing-valve mechanism, the valve leading downstream directly after the activatable swing-valve mechanism into the fresh gas line. This reliably ensures that the exhaust gas conducted into the fresh gas line is thoroughly mixed with the fresh gas.
The swing-valve mechanism is advantageously a butterfly valve. The amount of gaseous medium fed in can be controlled in a particularly simple manner by means of a butterfly valve.
The internal combustion engine is advantageously a direct injection spark ignition engine or a direct injection diesel engine. Systems for direct injection engines, i.e. spark ignition and diesel engines, are high-rate systems with severe sooting tendency. The condensate which arises encourages the valve to adhere in the seat after the internal combustion engine is switched off. With the above-described exhaust gas recirculation device, the risk of the valve adhering to its seat is particularly small.
The advantages obtained by the invention reside, in particular, in the fact that a fixedly predeterminable idle position is specified within or outside the movement range for the closing element. The use of the trailing effect of the control apparatus after the internal combustion engine is switched off enables the closing element to be moved into the idle position. An additional technical outlay on the internal combustion engine for the closing element of the valve to be moved into the idle position is therefore not required. Adhering or sticking of the closing element of the valve to its valve seat or to the exhaust gas recirculation line is particularly reliably prevented by the movement of the closing element of the valve into the idle position when the internal combustion engine is switched off. This means that the exhaust gas recirculation device is particularly low maintenance and at the same time has a particularly long service life.